1. Field of the Invention
The present invention relates to adhesives, more specifically adhesives for connecting a semiconductor chip to a substrate.
2. Background Art
Paste and film adhesives have been used to connect a semiconductor chip onto a substrate or a flexible wiring board.
Reference 101 in FIG. 3 represents an electric device comprising a semiconductor chip 111 bonded to a flexible wiring board 105 with an adhesive 112. Bump terminals 121 carried by the semiconductor chip 111 are in contact with metal lines 122 at the bottom of an opening 119 in the cover film 115 of the flexible wiring board 105.
The terminals 121 on the semiconductor chip 111 are connected to its internal circuit not shown, so that the internal circuit of the semiconductor chip 111 and the metal lines 122 on the flexible wiring board 105 are electrically connected each other via the terminals 121 in the state shown in FIG. 3.
The adhesive 112 has been cured, whereby the semiconductor chip 111 and the substrate 113 are also mechanically connected each other via this adhesive 112.
If the adhesive 112 is used, the semiconductor chip 111 and the flexible wiring board 105 can be thus connected without using solder.
However, the flexible wiring board 105 may be curved in the parts between terminals 121 on the semiconductor chip 111 as shown in FIG. 3, when the conventional adhesive 112 is used. When the flexible wiring board 105 is curved, the internal stress may become uneven, which may result in a short circuit between the flexible wiring board 105 and the semiconductor chip 111 or connection failure after aging.
When the adhesive 112 used for connection is not a film but a paste, air tends to be included in the adhesive 112 during the process of applying the adhesive 112 or heating under pressure, whereby voids may be generated in the adhesive 112 between the semiconductor chip 111 and the flexible wiring board 105. Reference 130 in FIG. 3 represents voids generated in the adhesive 112.
If many voids 130 are present in the adhesive 112, when the electric device 101 has been left to stand for a long time (aging) under high temperature ambience, the semiconductor chip 111 may float from the adhesive 112 on the substrate 113 to invite connection failure in the electric device 101.
What is needed, therefore, is an adhesive that less likely to cause voids in the adhesive and deformation of adherends when it is used for connection.
It has been discovered that curved xe2x80x9cadherendsxe2x80x9d result from the uneven residual stress in adhesives resulting from uneven distribution of the adhesives on the adherends after curing.
After studies to prevent the phenomenon described above, it was discovered that when the specific gravity of adhesives is adjusted to 1.4 or more and 4.0 or less by adding a filler having a specific gravity of 3.0 or more and 9.0 or less, the adhesives are not excessively pushed aside during heating under pressure, whereby inclusion of voids or curved adherends can be prevented.
In one aspect, therefore, the present invention provides an adhesive comprising a binder based on a thermosetting resin, a hardener for the thermosetting resin and an insulating filler, wherein the filler has a specific gravity of 3.0 or more and 9.0 or less and the adhesive has an overall specific gravity of 1.4 or more and 4.0 or less.
In one embodiment, the present invention provides the adhesive described above wherein the filler is contained in an amount of 5 parts by volume or more on the basis of the total volume of the binder and the hardener representing 100 parts by volume.
In one embodiment, the present invention provides the adhesive described above wherein the filler is contained in an amount of 35 parts by volume or less on the basis of the total volume of the binder and the hardener representing 100 parts by volume.
In one embodiment, the present invention provides the adhesive described above wherein the thermosetting resin contained in the binder is an epoxy resin.
In one embodiment, the present invention provides the adhesive described above containing conductive particles and having a specific gravity of 1.0 or more and 4.0 or less when the conductive particles are included.
In one embodiment, the present invention provides the adhesive described above wherein the conductive particles comprise resin particles and a metal coating formed on the surfaces of the resin particles.
In one embodiment, the present invention also provides an adhesive film obtained by forming a film of an adhesive comprising a binder based on a thermosetting resin, a hardener for the thermosetting resin and an insulating filler, wherein the filler has a specific gravity of 3.0 or more and 9.0 or less and the adhesive has an overall specific gravity of 1.4 or more and 4.0 or less.
In one embodiment, the present invention also provides an electric device comprising a semiconductor chip and a substrate with an adhesive being inserted between the semiconductor chip and the substrate and cured by heat treatment, wherein the adhesive comprises a binder based on a thermosetting resin, a hardener for the thermosetting resin and an insulating filler, wherein the filler has a specific gravity of 3.0 or more and 9.0 or less and the adhesive has a specific gravity before hardening of the adhesive, 1.4 or more and 4.0 or less.
Advantageously, therefore, embodiments of the present invention provide adhesives having an overall specific gravity of 1.4 or more and 4.0 or less by including a filler having a specific gravity of 3.0 or more and 9.0 or less so that not only voids are less likely to occur in the adhesives during connecting an adherend or a substrate and a semiconductor chip but also the substrate is less likely to be curved after connection.
Thus, electric devices in which components are connected with adhesives formed in accordance with embodiments of the present invention have not only excellent appearance but also high connection reliability. As used herein, the xe2x80x9cspecific gravityxe2x80x9d means the ratio of the density of a material to the density of water. Adhesives having an overall specific gravity of 4.0 or less are easy to handle during application because they have a low overall viscosity.
The adhesive of the present invention may contain a coupling agent. A coupling agent like a silane coupling agent has a high affinity for both organic materials and inorganic materials. Therefore, if a coupling agent is added to the adhesive of the present invention, the affinity between the adhesive comprising organic materials like binder and adherends comprising inorganic materials like metal wiring or glass substrate is improved.
When a filler having a specific gravity of 3.0 or more and 9.0 or less is added in an amount of 5 parts by volume or more and 35 parts by volume or less on the basis of the total volume of the binder and the hardener representing 100 parts by volume, the overall specific gravity of adhesives can be adjusted to 1.4 or more and 4.0 or less.
The adhesives of the present invention may contain a filler having a specific gravity of 3.0 or more and 9.0 or less and a filler having a specific gravity of less than 3.0 such as silica in combination. However, fillers having a specific gravity of less than 3.0 are not preferred to be used alone because the overall specific gravity of the adhesives becomes low. Fillers having a specific gravity exceeding 9.0 are not preferred because they are readily precipitated in the adhesives.
The binder used in the adhesives of the present invention may be preferably based on an organic compound such as a resin and contains at least a thermosetting resin. If a liquid epoxy resin is used as the thermosetting resin, a pasty adhesive can be obtained without using any organic solvent which has an adverse environmental effect during the process of preparing the adhesive and the process of connecting adherends.
The types of conductive particles added to the adhesives of the present invention are not specifically limited and include metal particles such as nickel particles and gold particles, and metal-coated resin particles formed of resin particle having a coating of a metal such as gold, nickel or solder on their surfaces. Among these conductive particles, metal-coated resin particles generally have a specific gravity in the range of 2.0 or more and 3.0 or less, which is smaller than those of conductive particles formed of metal particles.
When conductive particles consisting of metal-coated resin particles as described above are added to adhesives having a specific gravity of 1.4 or more and 4.0 or less prepared by mixing a filler having a specific gravity of 3.0 or more and 9.0 or less and a binder, therefore, the overall specific gravity of the adhesives neither falls below 1.4 nor exceeds 4.0 after the conductive particles have been added.